How It Works
Traditional phone service (sometimes
called "Plain Old Telephone Service" or POTS)
connects your home or small business to a telephone company office
over copper wires that are wound around each other and called twisted
pair. Traditional phone service was created to let you exchange
voice information with other phone users and the type of signal used
for this kind of transmission is called an analog
signal. An input device such as a phone set takes an acoustic signal
(which is a natural analog signal) and converts it into an
electrical equivalent in terms of volume (signal amplitude) and
pitch (frequency of wave change). Since the telephone company's signaling
is already set up for this analog wave transmission, it's
easier for it to use that as the way to get information back and
forth between your telephone and the telephone company. That's why
your computer has to have a modem
- so that it can demodulate the analog signal and turn its values
into the string of 0 and 1 values that is called digital
information.
Because analog transmission only
uses a small portion of the available amount of information that
could be transmitted over copper wires, the maximum amount of data
that you can receive using ordinary modems is about 56 Kbps
(thousands of bits per second). (With ISDN,
which one might think of as a limited precursor to DSL, you can
receive up to 128 Kbps.) The ability of your computer to receive
information is constrained by the fact that the telephone company
filters information that arrives as digital data, puts it into
analog form for your telephone line, and requires your modem to
change it back into digital. In other words, the analog transmission
between your home or business and the phone company is a bandwidth
bottleneck.
Digital Subscriber Line is a
technology that assumes digital data does not require change into
analog form and back. Digital data is transmitted to your computer
directly as digital data and this allows the phone company to use a
much wider bandwidth for transmitting it to you. Meanwhile, if you
choose, the signal can be separated so that some of the bandwidth is
used to transmit an analog signal so that you can use your telephone
and computer on the same line and at the same time.
How DSL Work
